Abstract
BACKGROUND: The mechanisms responsible for the development of airway hyperresponsiveness in asthma are poorly understood. Adenosine levels are high in the lungs of patients with asthma, but a role for adenosine in the development of this cardinal feature of asthma has not been previously reported. OBJECTIVE: To determine the capacity of adenosine to induce airway hyperresponsiveness, and to investigate the mechanisms behind these effects of adenosine on airway physiology. METHODS: Wild-type C57BL/6 mice were exposed to aerosolized adenosine analog adenosine-5' N-ethylcarboxamide (NECA), and subsequent hyperresponsiveness to methacholine was investigated by measuring airway mechanics after anesthesia and tracheostomy. Similar experiments were conducted with A(1)-deficient, A(3)-deficient, and mast cell-deficient mice, as well as with mast cell-deficient mice engrafted with wild-type (wt) or A(3)(-/-) mast cells. The effect of NECA on methacholine-induced tension development in ex vivo tracheal rings was also examined. RESULTS: Exposure of wt mice to NECA resulted in the robust induction of airway hyperresponsiveness. NECA failed to induce hyperresponsiveness to methacholine in tracheal ring preps ex vivo, and NECA-induced airway hyperresponsiveness in vivo was not affected by the genetic inactivation of the A(1) adenosine receptor. In contrast, NECA-induced airway hyperresponsiveness was abolished in A(3) adenosine receptor-deficient mice and in mice deficient in mast cells. Reconstitution of mast cell-deficient mice with wt mast cells restored hyperresponsiveness, whereas reconstitution with A(3) receptor-deficient mast cells did not. CONCLUSION: Adenosine induces airway hyperresponsiveness indirectly by activating A(3) receptors on mast cells.